Seamless Steel Pipe Composition for SSC Resistance
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Solution Overview
Problem
Existing methods for manufacturing high-strength seamless steel pipes with excellent sulfide stress corrosion cracking resistance (SSC resistance) for oil country tubular goods are insufficient, particularly for yield strengths above 110 ksi, as they struggle to stabilize carbide compositions and non-metal inclusions, leading to inadequate SSC resistance in corrosive environments.
Innovation Solution
A high-strength seamless steel pipe composition with specific elemental ranges (C: 0.20-0.50%, Si: 0.05-0.40%, Mn: 0.3-0.9%, etc.) and a microstructure with a tempered martensitic phase and controlled segregation index (Ps < 65) is developed, along with a manufacturing process involving heating, hot working, quenching, and tempering treatments to enhance SSC resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional manufacturing methods are used to achieve high yield strength (110 ksi or more), then strength requirement is met, but sulfide stress corrosion cracking resistance (SSC resistance) is insufficient
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.20-0.50%, Si: 0.05-0.40%, Mn: 0.30-0.90%, Cr: 0.60-1.70%, Mo: 0.40-1.00%, V: 0.010-0.300%, Nb: 0.010-0.060%, B: 0.0003-0.0030%) and heat treatment parameters (quenching temperature, tempering temperature, cooling rate) to simultaneously achieve high yield strength (758 MPa or more) and excellent SSC resistance. This systematic parameter optimization resolves the contradiction between strength and corrosion resistance.
Solution Approach 2:
The patent creates a composite microstructure consisting of tempered martensite as the primary phase with controlled carbide precipitates (MC type, M23C6 type, and M6C type) and refined grain structure. This composite microstructure, achieved through specific alloying and heat treatment, provides both the required mechanical strength and enhanced resistance to sulfide stress corrosion cracking.
2Reliability
If carbide composition is adjusted to improve SSC resistance, then SSC resistance improves, but manufacturing stability becomes difficult
Solution Approach 1:
The patent establishes specific parameter ranges for alloying elements that directly control carbide composition and distribution. By constraining C (0.20-0.50%), Cr (0.60-1.70%), Mo (0.40-1.00%), V (0.010-0.300%), Nb (0.010-0.060%), and B (0.0003-0.0030%) within defined ranges, the patent ensures stable formation of beneficial carbide types (MC, M23C6, M6C) while maintaining manufacturing process stability and repeatability.
Solution Approach 2:
The patent implements feedback control through strict compositional specifications and microstructural requirements (prior austenite grain number ≥8.5, tempered martensite volume fraction ≥95%, segregation index Ps <65). These feedback criteria guide the manufacturing process to consistently produce the desired carbide composition and microstructure, ensuring both SSC resistance and manufacturing stability.
3Reliability
If non-metal inclusions are controlled to improve SSC resistance, then SSC resistance improves, but manufacturing complexity increases
Solution Approach 1:
The patent controls non-metal inclusions through parameter optimization of alloying elements, particularly B (0.0003-0.0030%), Ti (0.005-0.030%), and Al (0.005-0.100%), which influence inclusion formation and characteristics. By adjusting these parameters within specific ranges, the patent achieves favorable inclusion properties (reduced harmful inclusions, optimized distribution) without requiring excessively complex manufacturing processes.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution achieves a yield strength of 758 MPa or more with excellent SSC resistance, maintaining both high strength and toughness, and is cost-effective for industrial applications.
Implementation Method 1
a microstructure where a volume fraction of a tempered martensitic phase is 95% or more
Implementation Method 2
heating, hot working, quenching, and tempering treatments
Implementation Method 3
quenching treatment in which the steel pipe is reheated at a temperature which falls within a range of AC3 transformation temperature or above to 1000° C. or below and the steel pipe is rapidly cooled
Implementation Method 4
after the quenching treatment, tempering treatment which heats the steel pipe to a temperature which falls within a range of 600 to 740° C. is performed
Data Source
AI summary
Provided is a high-strength seamless steel pipe having the composition which contains, by mass %, 0.20 to 0.50% C, 0.05 to 0.40% Si, 0.3 to 0.9% Mn, 0.015% or less P, 0.005% or less S, 0.005 to 0.1% Al, 0.008% or less N, 0.6 to 1.7% Cr, 0.4 to 1.0% Mo, 0.01 to 0.30% V, 0.01 to 0.06% Nb, 0.0003 to 0.0030% B, and 0.0030% or less O (oxygen). The high-strength seamless steel pipe has the microstructure where a volume fraction of a tempered martensitic phase is 95% or more, and prior austenitic grains have a grain size number of 8.5 or more, and a segregation degree index Ps which is defined by a formula Ps=8.1 (XSi+XMn+XMo)+1.2XP relating to XM which is a ratio between a segregated portion content and an average content is set to less than 65.